This invention relates generally to an apparatus which provides an electrical analog of the separation between the tips of a set of turbine blades and the housing in which the blades rotate. A number of noncontacting inspection equipments have been developed. Examples of the equipments which have been disclosed in United States Patents are as follows: U.S. Pat. Nos. 3,504,279 of Foster, et al; 3,641,431 of Pigage; 3,467,358 of Zablotsky, et al; 3,400,331 of Harris; 3,631,430 of West; 3,386,031 of Able, et al; 3,213,360 of Cook, et al; and 3,134,445 of Hotchkiss.
Many of the above mentioned devices and equipments sense proximity by means of capacitance. In Harris (U.S. Pat. No. 3,400,331) for example a change in capacitance at the sensing probe is used to create a frequency change in an oscillator, means are then provided for displaying the change in frequency. In Pigage (U.S. Pat. No. 3,641,431) a capacitance probe measures the air gap formed between two plates, one plate being the fixed probe, the other plate being the movable cutter blade element under test. The Pigage approach requires the use of a slip ring or other low resistance contacting element between the test instrument and the unit being checked.
The apparatus of Foster, et al. (U.S. Pat. No. 3,504,279) allows non-contact inspection of multistage turbine blades in a specially instrumented fixture. There is no provision for checking blade clearance directly in the operating housing. The vibration checker of Zablotsky, et al. utilizes two non-contacting sensors for checking the amount of root-to-tip oscillation in a turbine blade.
U.S. Pats. Nos. 3,386,031 and 3,134,445 deal with systems for checking the performance of helicopter rotor blades. The first mentioned of the two (U.S. Pat. No. 3,386,031) makes use of a capacitance element in which the propeller rotor comprises one side of the capacitor and a plate probe comprises the other element. Variations in capacitance are used to change the frequency of an oscillator in a manner generally like that of Harris (U.S. Pat. No. 3,400,331). The inspection system of Hotchkiss (U.S. Pat. No. 3,134,445) is implemented by pressure sensing means.
The electronic gauging system of Cook, et al. (U.S. Pat. No. 3,213,360) utilizes a differential capacitor comprised of three coaxial cylindrical plates. Two of the plates are the same diameter and they are placed end to end with a small gap therebetween. The third plate is of smaller diameter than the others. It is axially movable and is placed within the gap between the first two plates. The two like plates of the capacitor are incorporated in the arms of a balanced bridge. Motion of the third plate unbalances the bridge.
The position responsive apparatus of West (U.S. Pat. No. 3,631,430) matches a calibrated adjustable voltage to the output voltage from a pickup device. Both capacitive and inductive potentiometers are examples of the pickup devices used by West.
None of the above have the capability provided by my invention. My invention makes use of a balanced input which avoids the need for a ground return path of doubtful integrity through rotating machinery. Further, the circuitry enables as much as 100 feet of cable between the sensor probe and the signal processor without any degradation of results.